Printed circuit boards are widely used in the electronics industry for mounting and interconnecting various system components, such as integrated circuits, resistors, capacitors, inductors, and the like. Printed circuit boards are manufactured in a multi-step process, including a drilling step where holes for mounting of the system components are drilled in a preselected pattern on the board. As might be expected, precise location of the holes is critical, and even a single misaligned hole can render a board useless. Although the holes are typically drilled by numerically controlled drilling machines, power surges and other factors can sometimes affect drilling accuracy, and it is therefore necessary to check at least a statistical sampling of the boards.
The most common method for checking employs a master overlay sheet having the desired pattern of holes drilled in the sheet. By sandwiching the overlay and the printed circuit board together and holding the assemblage up to the light, an operator can detect misaligned holes by observing where the holes in the sheet and the board do not correspond. The printed circuit board may include thousands of drilled holes, and it is necessary to be able to consistently detect even a single misalignment. Moreover, it is imperative that the comparison time be minimized in order to reduce the cost of this quality check procedure.
Heretofore, master overlay sheets have been composed of a wide variety of materials, typically dimensionally stable translucent plastics. One such material, available from Polymer Plastics Corporation, assignee of the present application, has been sold under the trade name Trans-Chek. Trans-Chek is available in white, amber, and green, and it is composed of 0.01 inch polyvinyl chloride plastic sheets having a matte finish on both surfaces The green material, although generally functional, had a light transmissivity of less than 20% and has been found not to provide sufficient contrast between sheet material and the copper surface of the printed circuit board to enable operators to scan the printed circuit boards as rapidly as would be desirable.
For the above reasons, it would be desirable to provide improved plastic overlay materials for use in checking hole alignment on printed circuit boards.